The present invention relates to a low-cost device for power measurement of a plurality of motors. More particularly, the invention relates to a power measurement device for controlling motors, such as for example motors for driving grinding wheels used in machines for working glass, marble, et cetera.
It is known that in order to effectively control each motor of an apparatus, which in turn controls a working element of any sort, it is necessary to control in a substantially accurate manner the electric power that is supplied to the motor and is converted, minus losses, into mechanical power.
In electrical power measurement devices, this is normally done by measuring, for each motor, the current related to one or more phases (if a three-phase system is used) and the xe2x80x9cstarxe2x80x9d voltage that corresponds to those given phases or to two connected phases, subsequently measuring the phase shift between the voltage and the current.
Substantially, in addition to measuring the effective values of these currents and voltages, the zero crossings of said currents and voltages are detected.
The measurement of the phase shift between the voltage and the current for each motor, for example, is obtained by means of counters located on an appropriate clock (which emits pulses at a given frequency), with counter setting/resetting activated by the zero crossings of the measured current and measured voltage, so as to measure the phase shift between the voltage and the current.
Substantially, one measures the distance between a zero crossing of the current or voltage and a subsequent zero crossing (for example at the end of the period of the voltage or current), counting the intervening clock pulses.
The device that provides this is capable of giving a highly accurate measurement of the power supplied to the motor, also by using a microprocessor that accordingly receives in input the effective voltage and current values together with the zero crossing values of the voltage and current, in order to then calculate the phase shift between the voltage and the current.
However, the devices that provide the above described power measurement are expensive from the point of view of components and therefore for many applications that do not require very high measurement precision they are in a way xe2x80x9coversizedxe2x80x9d and accordingly increase needlessly the cost of an apparatus.
The aim of the present invention is to provide a power measurement device for motors and the like that is capable of providing a measured power value that is in any case sufficiently precise to control the motor but has a lower cost than known devices.
Within this aim, an object of the present invention is to provide a motor power measurement device that is capable of using a microprocessor with a firmware whose complexity is reduced considerably with respect to the firmware of microprocessors used in known devices.
Another object of the present invention is to provide a motor power measurement device in which the calculation of the phase shift between the voltage and the current of each motor is performed in a simpler way than in known devices.
Another object of the present invention is to provide a power measurement device for motors that is highly reliable, relatively simple to provide and at competitive costs.
This aim and these and other object that will become better apparent hereinafter are achieved by a device for measuring the power of a motor, comprising means for measuring the current sent to the motor and means for measuring the voltage, related to said current, that is sent to the motor, characterized in that it comprises rephasing means suitable to rephase the waveform of said voltage measured by said voltage measuring means and multiplier means suitable to multiply said current measured by said current measuring means by said voltage rephased by said rephasing means, the output signal of said multiplier means being processed in order to obtain the active power.